Merge bitcoin/bitcoin#30451: depends: remove Darwin ENV unsetting

bda537f7c4 depends: remove ENV unsetting for darwin (fanquake)
1807760f09 guix: improve ENV unsetting for macOS (fanquake)
0b2aeee21d depends: patch explicit -lm usage out of Qt tools (fanquake)

Pull request description:

  Now that we use the native compiler, and have fixed Qt, and these vars
  are (almost) unset in Guix, we can remove the unsetting from our compiler
  command here.

  I couldn't manage to make a darwin-clang-cross only exclusion of `-lm` work properly
  for Qt, so opted for just removing the explicit link entirely. I do not think this should have
  any other unwanted side-effects.

  Fixes #21552.

ACKs for top commit:
  TheCharlatan:
    ACK bda537f7c4

Tree-SHA512: 97a2d85de7d4b1d65717ecb521399ecba5f53863b8aef21af62ede5ceee59ee1a9392663da3a3852cad1b6d8b420dd4b0b5f0eea38d30a81785d8b2718620b5f

contrib/guix/libexec/build.sh

@@ -71,11 +71,9 @@ unset OBJCPLUS_INCLUDE_PATH
 
 export C_INCLUDE_PATH="${NATIVE_GCC}/include"
 export CPLUS_INCLUDE_PATH="${NATIVE_GCC}/include/c++:${NATIVE_GCC}/include"
-export OBJC_INCLUDE_PATH="${NATIVE_GCC}/include"
-export OBJCPLUS_INCLUDE_PATH="${NATIVE_GCC}/include/c++:${NATIVE_GCC}/include"
 
 case "$HOST" in
-    *darwin*) export LIBRARY_PATH="${NATIVE_GCC}/lib" ;;
+    *darwin*) export LIBRARY_PATH="${NATIVE_GCC}/lib" ;; # Required for qt/qmake
     *mingw*) export LIBRARY_PATH="${NATIVE_GCC}/lib" ;;
     *)
         NATIVE_GCC_STATIC="$(store_path gcc-toolchain static)"
@@ -180,6 +178,14 @@ make -C depends --jobs="$JOBS" HOST="$HOST" \
                                    x86_64_linux_NM=x86_64-linux-gnu-gcc-nm \
                                    x86_64_linux_STRIP=x86_64-linux-gnu-strip
 
+case "$HOST" in
+    *darwin*)
+        # Unset now that Qt is built
+        unset C_INCLUDE_PATH
+        unset CPLUS_INCLUDE_PATH
+        unset LIBRARY_PATH
+        ;;
+esac
 
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